Stockfish is the state of the art for its particular use case. General-purpose hash table are completely irrelevant, they'll never be able to compete on performance. Specific things Stockfish can assume:
* The table doesn't need to dynamically grow.
* Entries are allowed to be lost, but inserts can also simply fail
* Key collisions are considered acceptable, and because of this, Stockfish doesn't store the key in the table, only its hash. Even artificially high collision rates have been shown to not significantly impact playing strength.
* Bogus data due to race conditions is considered acceptable, same as above.
* The table doesn't need to dynamically grow.
* Entries are allowed to be lost, but inserts can also simply fail
* Key collisions are considered acceptable, and because of this, Stockfish doesn't store the key in the table, only its hash. Even artificially high collision rates have been shown to not significantly impact playing strength.
* Bogus data due to race conditions is considered acceptable, same as above.